docs(changelog): track playlist re-download (dupe drift) + no-duration wrong-cover bugs

Duplicate check matches exact computed path only, so iTunes/romaji/trackNum/
album drift between runs re-downloads existing tracks. The no-duration cascade
branch lacks Mittelweg protection and grabs wrong covers (Yorushika 車窓 →
Piano Echoes Shasou).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -238,3 +238,13 @@ Versioning: two-part `MAJOR.MINOR` — each release bumps the minor (features an
   Album-Artist field (and iTunes autofill fills no artist at all). Multi-artist lookup is also
   inconsistent: iTunes splits the artist on `; , &`, MusicBrainz on `, ; & •`, neither handles
   `/`, `feat.`, `ft.`. Both should use one robust "first artist only" splitter for the search.
+- **Backend (download)** — Re-downloading a playlist re-fetches some already-present tracks: the
+  duplicate check matches the exact computed path only (same album folder, exact track-number
+  prefix, exact resolved artist/title). Any drift between runs — different iTunes/romaji name, a
+  different track number, a different album folder, or a `- Piano Version`/`(var)` variant suffix
+  — makes the file look new. Should match by normalized first-artist + title across the artist's
+  folders, not the exact path.
+- **Backend (download)** — The no-duration branch (tracks where iTunes returns no duration) has
+  **no Mittelweg protection**: it still falls through to title-only / no-filter grabs and pulls
+  wrong covers by other uploaders (observed: `Yorushika – 車窓` saved as `Piano Echoes – Shasou`).
+  The channel-required relaxation should apply to the no-duration branch too.